This invention relates to electrophotographic processes and apparatus. In one of its more particular aspects this invention relates to a multi-layered ion modulator and its use in improved electrophotographic processes.
Electrophotographic reproduction techniques for making copies of graphic originals using photoconductive media are well known. Such processes generally call for applying a blanket electrostatic charge to a photoconductor in the dark and then exposing the charged photoconductor to a pattern of light and shadow created by directing electromagnetic radiation onto a graphic original. The light-struck areas of the photoconductor are discharged leaving behind a latent electrostatic image corresponding to the original. A developed image is produced by applying an electroscopic powder to the latent electrostatic image and then fixing the image or transferring and fixing onto a suitable receiving medium such as plain paper.
This technique has been extended to foraminated structures which are formed by applying a photoconductive layer to a conductive screen or similar apertured structure. Such structures function as ion modulators selectively passing a stream of ions through the apertures of the screen in a pattern corresponding to the graphic original to be reproduced.
The ion modulators which have been developed heretofore and are known in the prior art fall into several distinct classes:
The first is a simple two-layered screen or grid construction which is formed by applying a photoconductive layer onto an apertured metallic substrate as disclosed in U.S. Pat. No. 3,220,324 to Christopher Snelling. Such a structure can be used to apply to a dielectric target an electrostatic charge corresponding to a pattern of light and shadow created by electromagnetic radiation directed onto a graphic original resulting in the creation of a latent electrostatic image thereon. The majority of the ions are propelled through the areas of the screen which were in a shadow or dark region of the projected image, but some fraction of the ions creating the latent electrostatic image are allowed to pass through the apertures in areas of the modulator corresponding to light or background areas in the original. Copies which are made from electrostatic latent images created in this manner therefore do not generally display the high contrast and low background levels desired.
A second group of photoconductive screens of various constructions has been adapted for use with charged material particles such as charged electroscopic powders. Typical of such screens are those disclosed in U.S. Pat. Nos. 3,694,200 to Gerald L. Pressman and 3,713,734 to Hewitt D. Crane, Gerald L. Pressman and George J. Eilers. Such structures suffer from the deficiency that charged particles accumulate in those areas of structure which attract the particles so that, ultimately, it is required that the screen be cleaned to physically remove the particles in order that the screen may be reused.
The use of such screens with ions rather than charged electroscopic powders is disclosed in U.S. Pat. No. 3,645,614 to Samuel B. McFarlane, Jr., Joseph Burdige and Norman E. Alexander. However the process involved in the use of these screens requires a multiplicity of steps and results in a reversal image.
While the prior art modulators have advanced the electrophotographic art, there are disadvantages which need to be overcome in order to provide an ion modulator system which can be operated so that copies of high quality can be made.